How this calculation works
Start with the geometric volume of a rectangular slab, then add your chosen allowance. All dimensions must be in metres. Thickness of 100 mm is 0.1 m.
Order volume = length × width × thickness × (1 + allowance/100)A worked example
A 5 m by 4 m slab, 0.1 m thick, needs 2 m³ geometrically or 2.2 m³ with 10% allowance.
Good to know
Quantity estimate only, not a mix design or structural specification. Confirm delivery increments, site conditions and ordering allowances with the supplier.
A small thickness change affects the entire slab
For a 5 by 4 m slab, every extra centimetre of thickness adds 0.2 m³. That comes from 20 m² multiplied by 0.01 m. Thickness can look small on a drawing while having a noticeable effect on a concrete order.
The allowance is applied to the complete geometric volume, not to each dimension. Increasing all three dimensions by 10% would increase volume by 33.1%, a different operation from adding a 10% quantity allowance.
Measure, convert and confirm before ordering
All dimensions must be in metres: 100 mm thickness means 0.1 m. Check formwork dimensions, uneven ground and any separate footings or thickened edges. A rectangular slab formula cannot infer those features.
The result estimates quantity, not strength, reinforcement or mix proportions. Ask the supplier about delivery increments and confirm a project-specific allowance rather than treating the default percentage as a construction standard.
A few questions, answered
Does the allowance change slab thickness?
No. It increases the estimated order quantity after calculating the entered geometric volume.
Does this specify a structural slab design?
No. Thickness, reinforcement and mix design require appropriate project specifications.